Electronic throttle control (ETC) systems position the throttle plate in response to a throttle plate position command from the engine control unit (ECU). One such system is disclosed in U.S. patent application Ser. No. 554,178 entitled "Method and System for Engine Throttle Control" filed Nov. 6, 1995, assigned to the assignee of the present invention, and incorporated herein by reference. All ECU plate position commands are referenced to the position at which the plate is closed in the throttle bore. In this position the throttle plate sector gear impinges upon a throttle return control screw (TRCS). The TRCS serves as an adjustment at the factory to set the plate closed position. TRCS is therefore the absolute zero throttle plate reference position for all ECU plate position commands. In order to accurately position the plate, the ETC control system must then reference all of its actions to the TRCS.
The ETC uses an electric motor to position the plate. The motor is physically connected to the plate shaft via a single stage spur gear train, consisting of a pinion gear which is ground into the end of the motor shaft and a mating sector gear which is attached to the plate shaft. Due to the limitations of mass production machinery, the gear train always has a certain amount of backlash, or "lost motion" from the input (pinion gear) to the output (sector gear). Backlash is the amount of pinion gear angular displacement required to move a pinion gear tooth from a position just engaging the sector gear tooth in the plate closing direction, to a position just engaging the adjacent sector gear tooth in the plate opening direction.
To accurately position the plate, the control system must have accurate information on the position of the motor shaft and the plate. Plate position can either be directly measured via a sensor located at the plate shaft, or inferred via a sensor located on the motor shaft. It is generally more economical to use a motor shaft position sensor to measure both motor and plate position. Accurate measurement of plate location by the motor shaft position sensor then depends on a known relationship between motor shaft rotation and plate rotation. Using a priori knowledge of the transfer function of an ideal (zero backlash) gear train, the ETC control system can infer plate position from the measured motor shaft position.
To establish the initial positional relationship (also known as absolute zero throttle plate reference position calibration) between the motor shaft angle and the plate angle, the plate must be moved to a known location such as TRCS, and the motor shaft position sensor interrogated. This value of position is then assigned as the absolute zero throttle plate reference position for the ETC control system. However, backlash produces a region of uncertainty between the actual plate location (at TRCS) and the inferred plate location from the motor shaft position sensor. This region of uncertainty corresponds exactly to the width of the backlash region, measured in degrees of motor shaft angular position.
In order to remove this uncertainty, the motor shaft pinion tooth should be engaged in the throttle opening direction, with the sector gear, when the absolute zero throttle plate reference position calibration measurement is performed. One method of achieving this objective puts the ETC in an open-loop mode, i.e., ignores the throttle position command and the throttle position. Instead, the ETC commands an amount of torque equals to 1/2 of the torque pre-load value for a short period of time in order to move the motor shaft in the opening direction but not the throttle plate. After the system settles down at the end of the period, it is assume that the motor shaft's pinion gear is in direct contact with the sector gear on the throttle shaft in the opening direction. The ETC stores this position (absolute zero throttle plate reference position) in memory and uses it for actual throttle position computation. The major disadvantage of this method is that it can only be executed when the engine is not running. Therefore, it is not practical for normal operation, but used only in the ETC assembly plant. Subsequent to assembly, shifts in the absolute zero throttle plate reference position occur as a result of aging, wear, and thermal expansion/contraction causing erroneous computation of actual throttle plate position.